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Because general anesthesia paralyzes the body, a tube is placed in the patient's throat to ventilate the lungs during surgery. If the patient vomits and there is food in his stomach, there is a chance for the food to go into the lungs instead of the throat, according to About Health contributor Jennifer Heisler. Inhaling stomach contents into the lungs increases the risk of developing pneumonia and other respiratory complications. If it is necessary to take oral medication the day of the surgery, the medication should be taken with the smallest amount of water possible.

If a patient is having gastrointestinal surgery, the presence of food also increases the risk of infection and surgical complications. This is another reason to avoid eating before surgery, according to Dr. Wade. Patients undergoing gastrointestinal surgery usually need to drink clear liquids for at least one day prior to their scheduled procedures. Prior to colon or rectal surgery, it is also necessary to empty the bowel completely by taking magnesium citrate or by using another type of bowel preparation, as indicated by Brigham and Women's Hospital.